1. Field of the Invention
The present invention relates to solar cells and other photovoltaic devices formed by the junction of two photovoltaic materials of different semiconductor conductivity type with an overlying metalized grid layer, and processes for making such photovoltaic devices and apparatus. The present invention relates particularly to a photovoltaic apparatus utilizing relatively wide conductor grids or grid lines on the surface of the device intended to be exposed to radiant energy, such as sunlight.
2. Description of the Prior Art
Photovoltaic devices such as solar cells utilizing layered junctions of semiconductor materials, such as crystalline silicon, are known in the art, and examples are shown in Gibbons U.S. Pat. No. 4,434,318 and Delahoy U.S. Pat. No. 4,849,029. A figure of merit commonly associated with such photovoltaic devices is the efficiency of converting the incident radiant energy into electrical energy. In turn, the efficiency is directly related to the open circuit voltage achievable with the device together with the "fill factor," as would be understood by those skilled in the art.
The prior art has recognized problems with conventional solar cells caused by the high surface recombination velocity of the photovoltaic materials. This phenomenon can degrade the open circuit voltage and fill factor of the photovoltaic device.
Various solutions to this problem have been proposed, including the construction shown in U.S. Pat. No. 4,589,191 to Green et al. In the Green et al. device, a thick passivating oxide layer is formed between the upper photovoltaic layer (relative to the incident radiant energy) and the overlying metalized grid layer. Small discrete apertures or openings are formed in the oxide layer to provide direct contact between the metalized grid layer and the upper photovoltaic material layer. Also, the thickness of the passivating oxide layer is controlled to provide indirect conductivity between the upper photovoltaic material layer and the metalized grid layer via the "tunneling" phenomena.
While constituting improvements over the other state of the art photovoltaic devices, these structures require relatively complicated process steps and controls and result in devices with "shadowed" junctions, that is, semiconductor junctions existing under the metalized grid layer which, although shadowed from at least a portion of the incident radiant energy, can nevertheless act as recombination sites. These "shadowed" junctions can lead to a deterioration in the performance for photovoltaic apparatus constructions utilizing relatively wide grid lines, such as cells designed for use with a prismatic cover.